SSEClient stored any id it received, but the client rejects a request
whose header value is invalid. An id with a NUL, a control character or
surrounding whitespace therefore made every reconnect fail with
Error::InvalidHeaders before reaching the server, and the id could
never be replaced.
Accept an id only if it is a valid header value, so that the previous
id stays in effect. The SSE spec asks for this for NUL only; the rest
follows from what this client can send.
yhirose
5189edc52e
